1. Field of the Invention
This invention relates to contact lenses including those contact lenses having a predeterminable normal lifetime. In particular, the present invention relates to methods useful to quickly and effectively identify lense whose wear-life approaches the recommended lifetime of the lens.
2. Related Art
Currently, a major problem with disposable lenses is that patients do not dispose of the lenses pursuant to the manufacture's instructions, that is, by the time the manufacturer believes that the lens has reached its lifetime and has become overworn and should be discarded. Patients often do not dispose of the lenses when the manufacturer indicates they have run their lifetime since many patients perceive lenses to be functional and safe past the recommended wear schedule. This can cause problems with regards to lens performance and the health of the patient due to the fact that an overworn disposable lens is very susceptible to deposit formation, and prolonged extended wear places the patient at a higher risk for ocular complications.
Contact lenses are often periodically disinfected to protect the wearer's eyes from infection and to improve the wearer's comfort. For example, U.S. Pat. No. 4,997,626 to Dziabo and assigned to Allergan, Inc. discloses a method for disinfecting a contact lens comprising contacting the contact lens to be disinfected in a liquid medium with chlorine dioxide present in an amount effective to disinfect the contact lens to be disinfected.
It is well known to apply tints to contact lenses for a variety of reasons. Opaque tints are used to change eye color. Cosmetic tints, also known as "enhancement" tints, are used to enhance the eye color. CIBA Geigy sells a number of cosmetically tinted lenses including those having blue, green and brown tints. Bausch and Lomb manufactures lenses having blue cosmetic tints, among other colors.
Both cosmetic tints and opaque tints are typically applied to the portion of the lens that would cover the iris of the eye.
Another type of tint is a "handling" tint. A handling tint is typically applied to the entire contact lens surface, but at much reduced concentration when compared to an enhancement tint. A handling tint is not intended to enhance or change eye color. Rather, a handling tint is intended to assist the lens user in locating lenses misplaced due to the transparency of the lens material. Due to the slight intensity of color, when the handling tinted lens falls to the bottom of a white lens case, the user can better observe the lens and thereafter handle it. Thus, a handling tint is used for simplicity and ease of lens "handling".
As can be appreciated from the above, the tinting of contact lenses and how the intensity and color of a tint can be varied by varying the amount of the tint in the lens is known. See for example U.S. Pat. Nos. 4,891,046 to Wittmann; 4,559,059 and 4,468,229 to Su; 4,777,684 to Johnson; and 4,252,421 to Foley, Jr. The Wittmann Patent and Su U.S. Pat. No. 4,559,059 note the tendency of the tint in contact lenses to fade or leach out over time or use.
It is known that chlorine has an effect on tinted hydrogel lenses, namely the bleaching of the lenses and the loss of color in the lenses. See "Effect of Chlorine on Tinted Hydrogel Lenses", M. Liebetreu, G. E. Lowther and G. Hammack, Int. Eyecare, 2(10): 525-30 (Oct. 1986).